1. Field of the Invention
The present invention relates to semiconductor integrated circuits and, more specifically, to a semiconductor integrated circuit provided with memory cells of a DRAM.
2. Description of Related Art
A DRAM (Dynamic Random Access Memory) has recently become popular for use as a storage device in a semiconductor integrated circuit. In the DRAM, data is recorded in storage nodes by storing electrical charge in a capacitor in each of the memory cells, and data is exchanged between bit lines and the storage nodes via a gate transistor. For performing data exchange as such without fail even if the data stored in the storage nodes has a voltage almost the same as a power supply voltage VDD in the DRAM, a step-up voltage VPP higher than the power supply voltage VDD is applied to the gate in order to bring the gate transistor to a selected state.
For such an operation, the gate transistor of the DRAM is required to have voltage resistance of a level enough to withstand an application of the step-up voltage VPP. With a MOS (Metal Oxide Semiconductor) transistor, the voltage resistance can be increased with a thicker gate oxide film. When the transistor does not have the voltage resistance high enough for an applied voltage, the element suffers age deterioration faster than usual, or element is damaged. In consideration thereof, the gate oxide film of the gate transistor generally has the same thickness as that of a transistor of a driving circuit supplying the step-up voltage VPP to the gate of the gate transistor.
On the other hand, a power supply voltage VDD is applied to the transistor constituting a control circuit, and such a transistor is not thus required to have the voltage resistance as high as the transistor of the driving circuit and the gate transistor. Accordingly, any transistor constituting a circuit operating with the power supply voltage VDD, e.g., control circuit, can have a gate oxide film thinner than those of the transistor of the driving circuit and the gate transistor. With a thinner gate oxide film as such, the circuit operation can become faster, and the transistor can be more miniaturized.
As such, with a semiconductor integrated circuit, the thickness of the gate oxide film (or the level of the voltage resistance) of the transistor is changed in accordance with an applied voltage, thereby being able to reduce the circuit size while assuring the level of the voltage resistance of the transistor. Japanese Unexamined Patent Application Publication No. 2001-15704 describes such a previous technology of using a plurality of transistors having different thickness in accordance with an applied voltage.
The present inventors have found a problem as follows. The transistor has two different types of voltage resistance, one is not to immediately damage elements after voltage application, and the other is not to cause age deterioration to elements due to continuous voltage application. As such, as for transistors to which a high voltage is applied, when any transistor for use in a portion to be subjected to short-term application of high voltage is so designed as to be resistant to continuous application of high voltage, the resulting transistor shows considerably small performance deterioration in view of the product life. With such a voltage-resistant design, however, the gate oxide film in the transistor will be excessively increased in thickness, thereby causing a problem of increasing the circuit size due to the increase in element size.